The mean power-level of an optical signal generator, also referred to as the optical power level herein, lies halfway between the power-level of the generator's output when an 0-bit is being transmitted and the power level of its output when a 1-bit is being transmitted. It is desirable to accurately control the mean power-level of a generator so as to set it to a desired value, and more particularly to control the mean power-level so as to set it to one of a range of power levels.
Some previous optical power controllers are implemented as variable power-level laser diodes. An example of this sort of controller is described in United Kingdom Patent Application 2,220,092A, published on Dec. 28, 1989, in the name of STC PLC. Such controllers react to discrepancies between the mean power-level and a desired power-level by adjusting the injection current of the laser diode. One problem with such systems however, is that the injection current cannot be significantly adjusted without adversely affecting the extinction ratio of the laser diode. This problem limits the range of power-levels supported by such power controllers, and requires the addition of an extinction ratio control loop to the design. Another problem with systems that adjust the injection current of the laser diode, is that the injection current cannot be tuned for wavelength locking purposes as it is being tuned to control optical power.
Other known controllers do not operate by adjusting the injection current going into the laser diode, and thus do not adversely affect the extinction ratio of the generator. Their operation involves having the laser diode generate a constant-power optical signal, and then varying that signal's mean power level using a voltage controlled optical attenuator (VCOA) positioned at the output of the laser diode. That is, such controllers react to discrepancies between the actual mean power level and a desired mean power level by adjusting the attenuation imposed on the signal by the VCOA.
Even these other systems however, have key problems. One problem is that the relationship between a desired change in the power-level of the VCOA output, and the change in the control signal voltage required to effect that power-level change, is complex. This makes it difficult to generate the required variety of control voltages without the use of some processing means for performing operations such as interpolation.
Another problem is that the relation between a desired change in the power level of the VCOA output which is specified in dBm units, and the change in the control signal voltage required to effect that power level change, which is expressed in volts, is logarithmic. If the former quantity is represented by the variable, y, and the latter by the variable, x, the relation can be described as follows: EQU y=10 log (Cx/1mW);
C is some constant number The logarithmic relation between x and y means that the power controller must generate, on one hand, high resolution control voltages that extend over a relatively small voltage range in order to attain all the smaller desired power levels. The relation means, at the same time, that the power controller must generate, on the other hand, low resolution control voltages that extend over a relatively large voltage range in order to attain the greater desired power levels.
At least one alternative optical power controller makes use of a VCOA while effectively avoiding these problems. Such a controller is taught by U.S. Pat. No. 4,927,266, by Sugiura et al. issued on May 22, 1990, and involves generating a plurality of control signals of varying resolutions. These control signals are used as inputs into one or more VCOAs tuned at varying resolutions, and a variable injection current source for driving a light source. Such systems however, require a control processor unit (CPU) to generate the plurality of control signals. This, in turn, drives up the cost of the power controller, as well as the amount of space it must occupy.
The system disclosed in U.S. Pat. No. 4,927,266 has further drawbacks. For example, it varies the injection current to achieve fine-resolution control over the optical power. As described earlier, this feature is undesirable as it rules out varying the injection current for wavelength locking purposes.
Therefore, at present, an optical mean power controller is required that can generate the required variety of control voltages for input into the VCOA at a sufficiently high resolution over a desired range, without needing to make use of a CPU.